A thread rolling gear according to the present invention is used for various kinds of industrial products typically represented by automobile parts. For example, techniques for developing hybrid vehicles and electric-powered vehicles have been rapidly advancing in the automobile industry. In particular, a demand for power-saving and small-sized electric actuators in all kinds will increase in future. Further, it is required that components of those actuators have good efficiency in power transmission and manufacture.
A number of gears are used in the actuators. Those gears have great influence on the functions and the manufacturing costs of the actuators. As a method of manufacturing the gears efficiently, a thread rolling method for thrusting cylindrical dies against a cylindrical solid work is generally known, for example. In such a method, the pair of cylindrical dies each having a profile of a desired gear that are provided to face each other across the work are allowed to thrust against the surface of the work, synchronized with each other in rotating speed and thrusting speed against the work, thereby to form the gear.
The thread rolling method using the dies uses linear rack dies or the cylindrical dies. In the thread rolling method using the rack dies, addendums formed in an end portion of the rack dies having a fixed length are brought into press contact with the work and then fed with the dies being pressed against the work, thereby to allow the work to roll. In many cases, a tooth depth in the rack dies is different between an area in an early stage of the thread rolling process where the addendums begin to contact the work and an area in a finishing stage of the thread rolling process. Therefore, in the thread rolling using the rack dies, the profile of the gear to be formed is influenced by the length of the rack dies.
On the other hand, in the case of using the cylindrical dies, the cylindrical dies are rotated and concurrently thrust against the work. Therefore, the tooth profile is constant in any portion of the cylindrical dies, which provides a simple arrangement in the cylindrical dies. In addition, the rotating direction of the cylindrical dies can be switched over during the thread rolling process, which is advantageously applied to the thread rolling of large-sized worms and helical gears.
An example of the conventional thread rolling method using the cylindrical dies is disclosed in a published document indicated below. Such a method provides a thread rolling apparatus for helical gears, and worms and screws (mechanical elements having a lead in a flank), in which “slippage” can be reduced while simplifying the arrangement of the apparatus. In this, the “slippage” indicates a phenomenon in which the work moves along rotation axes of the cylindrical dies when the cylindrical dies are thrust against the work. This phenomenon occurs when the relative relationship between the addendums of the dies and tooth grooves formed in the surface of the work in advance is not proper. More particularly, the addendums of the dies are thrust against the tooth grooves of the work in a distorted manner to cause the dies and the work to follow advance/return motion of a screw, as a result of which the slippage occurs. When the slippage occurs, a portion of the surface of the work that should not be threaded is threaded, which makes quality maintenance difficult.
In the known technique described above, a biting tooth, a forming tooth, a finishing tooth and the like are provided in an outer circumference of each of the cylindrical dies provided to face each other across the work to form worms in the work during one rotation of each cylindrical die. A chamfered portion is provided in an axial one end of the outer circumference of each cylindrical die to allow the forming tooth or a clearance tooth to follow the worms in response to the axial movement of the work in forming the worms. The above prior art document describes that such an arrangement can eliminate the disadvantages caused by the slippage phenomenon without controlling the drive of the cylindrical dies by a controller.